Big Yellow Asteraceae. This is the time of year when you are likely to see huge, yellow-flowered composites that are tricky to assign to genus. The thread will guide you through the various Tribes and Genera involved, and illustrate the commonent species you are likely to find.
The genera we're talking about exclude the shrubs or sub-shrubs like Dittrichia (left) or Brachyglottis (right) with woody stems. Out plants are all herbaceous and mostly more than 1m tall.
Again, we're not talking about Asteraceae where all of the flowers are ligulate (dandelion-like) as seen in some big yellow thistles like Scolymus hispanicus (left) or knapweeds like Centaurea macrocephala (right)
Most of the big yellow Asteraceae have alternate leaves (left), so we'll start with the genera that have all their leaves opposite (right) or opposite at the base and alternate at the top (i.e. some clearly opposite leaves on the flowering stem).
There are 4 genera of big yellow Asteraceae that have opposite leaves: Coreopsis, Dahlia, Guizotia and Helianthus (the last with alternate leaves on the upper stem). Does your plant have big underground tubers (n.b. you need land-owner's permission to uproot a plant)
If your plant has tubers you have either Dahlia (left above) or Helianthus tuberosus (right, above). Dahlia has pinnate or ternate leaves (left) while Helianthus has entire leaves (right)
There are loads of yellow Dahlia cultivars, but the ones that are likely to fox you are the single hybrids like D. 'Knockout'
The flowers of Jerusalem Artichoke (Helianthus tuberosus) are on exceptionally tall stems (up to 2.5m).
You have excavated the base of your opposite-leaved plant and failed to find any tubers. Look at the leaves. Are they divided to the base or to the midrib (left), or are they entire, toothed or only shallowly lobed (right). The divided leaves belong to the genus Coreopsis (left).
The genera with entire leaves (right, above) are told apart by their leaf bases: clasping at the base is Guizotia (left), no clasping (either petiolate or sessile) is Helianthus (right, a sessile, non-clasping example, from H. x laetiflorus)
The perennial sunflowers are told apart by their stems. If they are roughly pubescent all the way to the base you have Helianthus x laetiflorus (left), and if they are more or less hairless in the lower half of the stem you have H. x multiflorus (right).
Tall species of Coreopsis are increasingly popular in praire style planting schemes. All their leaves are opposite and variously dissected. The plants you are most likey to see are C. tripteris (TL), C. grandiflora (TR), C. verticillata 'Moonbeam' (BL) and 'Zagreb' (BR).
Most genera of big yellow Asteraceae have alternate leaves all the way to the base. We cover these next. The first thing to do is open up a flower-head (capitulum) and look closely at the top of the achene (the fruit = "seed"). Is the pappus of hairs (left) or not (right).
Next one is a bit more trciky. Remove several achenes so that you can see the surface of the receptacle (the 'bottom' of the capitulum, pitted with holes where the acghenes were seated). Is there scale next to each pit (illustrated) or not? These are the receptacular scales.
Let's begin the alternate-leaved genera of big yellow Asteraceae with those that have a pappus of conspicuous hairs. The first decision concerns the phyllaries (the involucral bracts). Are they in 1 row (left) or more than 1 row (right)
If the phyllaries are in one row you have either Ligularia (left) or Sinacalia (right)
Sinacalia has very sparse inflorescences (just 3 or 4 ray flowers and 3 or 4 disc flowers), non-sheathing petioles, and lower leaves that are deeply pinnately lobed. The only species you are likely to see is S. tangutica from China.
The 2 commonest Ligularia species are strikingly different. L. dentata (left) has big, kidney-shaped, toothed leaves and corymbose flowers, while L. przewalskii (right) has deeply palmately lobed lower leaves and a spike-like panicle.
Next, we deal with the genera that have 2 or more rows of phyllaries. If the bracts are in 2 distrinct rows of roughly equal length, you have Doronicum (left). If the bracts are unequal, you have something else (right)
The Doronicum you are most likely to see naturlised outside a garden is D. pardalianches
If the basal leaves of your Doronicum all have cuneate bases, then you have the much rarer D. plantagineum.
In gardens you are more likely to see the two hybrids, both of which tend to get mis-recorded as Doronicum plantagineum. D. x excelsum (illustrated) has bigger leaf teeth (>2mm) than D. x willdenowii, and acute rather than obtuse leaf tips.
The most important genus with a pappus of hairs and unequal phyllaries is Inula. The huge species is Inula helenium (illustrated); it is over 1m tall and has flower-heads more than 5cm across.
The smaller Inula species that have very narrow (long subulate) spreading phyllaries are I. hookeri (left) and I. orientalis (right). Both have long eglandular haris on leaf and stem, but only I. orientalis also has short glandular hairs on the stem and leaf blades.
The smaller Inula species with lanceolate, appressed (not subulate spreading) phyllaries are told apart by their leaves. I. salicina has prominent reticulate veins on the upper surface and hairless achenes; I. oculus-christi leaves are obscurely veined and the achenes are hairy.
So that's it for the alternate-leaved, big yellow Asteraceae that have a pappus of hairs: to recap that's Ligularia (TL), Sinacalia (TR), Doronicum (BL) and Inula (BR).
Now for the alternate-leaved, big yellow Asteraceae that do not have a pappus of hairs. The first group we shall deal with are those that do have receptacular scales.
If the receptabcle has abundant bristles and no scales, you have Gaillardia, otherwise something else. Most Gaillardia x grandiflora are zoned with 2 or 3 contrasting colours (left) but a few are all-yellow (e.g. G. 'Messa Yellow', right).
The other genera have normal receptacular scales (i.e. not bristle-tipped). If the achenes are strongly compressed and at least 2 times as wide as thick, with strong ribs or narrow wings, then you have an alternate-leaved Coreopsis (recall they are typically opposite leaved.
The 2 important genera in this group are Rudbeckia (left) and Buphthalmum (right). The ligules are broad in Rudbeckia (more than 3mm wide, left) and narrow in Buphthalmum (less than 3.1mm wide, right).
Rudbeckia are increasing popular as subjects for prairie planting. Most specacular of all is R. maxima with its towering flower stalks and stikingly broad, blue-green leaves.
Two of the most popular hardy prairie plants are Rudbeckia missouriensis (left) and R. fulgida var. deamii (right)
The 2 Rudbeckia species that are naturalised in Britian are told apart by their height and hairiness. The tall plant, R. laciniata, is almost hairless (left) with deeply divided leaves. The shorter, hairy plant is R. hirta (right) with almost entire leaves.
There are 2 frequently seen species of Buphthalmum. The huge plant, B. speciosum (which used to be called Telekia speciosa), has lower stem leaves that are petiolate and cordate. The shorter plant, B. salicifolium, has narrowly cuneate stem leaves and much broader ray flowers.
The last group of alternate-leaved, big yellow Asteraceae lack both hairy pappus and receptacular scales. Look underneath the leaf. If it is densely white-hairy (illustrated) you have Gazania, Arctotis or Arctotheca. Otherwise something else.
Gazania rigens has the outer phyllaries fused into a cup-like structure (left). The ray flowers are often striped (top right), but can be pure yellow (bottom right).
You tell Arctotis from Arctotheca by the achenes. In Arctotis (left), the outer achenes have 3 prominent ridges. Achenes in Arctotheca (right) are un-ridged and the ray flowers are sterile (i.e. they have no achenes).
The last group we need to deal with do not have white-hairy under-leaves. The most important of the genera in this group is Helenium (left). They have a pappus of 1-8 bristles (right), sometimes with minute scales as well.
Most of the Helenium cultivars you will see are red or orange. The most popular yellow cultivars are H. autumnale 'Helena Gold' (left) and H. 'Goldrausch' (right)
There are three other genera that sometimes have yellow flowers: Euryops pectinatus (main, an evergreen shrub), Ismelia carinata (typically with 3 bands of colour in the ray flowers, but here just 2; TR) and Leucanthemum 'Real Goldcup' (BR) a mutant of Shasta Daisy
So that's it. The crucial things to know are whether your plant has a pappus of hairs (e.g. Inula), whether it has receptacular scales (e.g. Rudbeckia), and whether it has opposite (e.g. Helianthus) or alternate leaves (e.g. Helenium). Let me know how you get on.
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